162 THE FUTURE OF ARID LANDS 
palynology, pedology, or dendrochronology, gives information 
along a prescribed line, whereas another approach gives different 
data. Through correlating and, or, superimposing one over the 
other(s), the climatic picture begins to evolve. 
Tree Rings and Relative Rainfall 
The first method I should like to discuss is dendrochronology. 
This method yields absolute dates on specific tree rings according 
to the growth year, and it yields relative rainfall patterns. There 
is, at present, no technique known in which tree-ring data can 
give information along the lines of absolute quantities. For ex- 
ample, we can establish departures from a running mean which in 
itself is the average of the data in hand; we can determine if a 
year, or a period of years, is below average, average, or above 
average in growth, but no method is yet known that will allow 
us to determine the absolute amount of rainfall represented by 
that average. Because many factors influence the growth of a tree, 
it is impossible to isolate completely one physiological or environ- 
mental factor from all the others. Thus we are not positive that 
we have a true or absolute representation of the rainfall patterns in 
contrast to patterns representative of other growth factors. 
In the course of our tree-ring studies, thousands of samples from 
living trees as well as historic and prehistoric specimens from over 
the southwestern United States have been analyzed. Working with 
the archaeological material supplied in quantity by southwestern 
excavators, we have developed regional tree-ring indices for the 
major river drainages and their related geographic regions in this 
general climatic area. These local chronologies extend into pre- 
historic times. They are limited by the material available; conse- 
quently their individual lengths of time vary from one region to 
another. The study of these regional indices is profitable because 
they indicate the less pronounced variability that has existed 
from one region to another within the larger and relatively homo- 
geneous climatic area. The tree-ring “droughts,”’ or periods of 
deficient rainfall as exhibited in tree growth, can be considered as 
“droughts” only inasmuch as they fall below the average. 
Aside from the relative quality of these deficient periods, there 
